An organic light-emitting display device does not require an additional light source unlike a liquid-crystal display device and thus can be made lighter and thinner. Further, an organic light-emitting display device has advantages in that it is driven with low voltage to consume less power, and in that it represents vivid colors, has short response time, wide viewing angle and good contrast ratio (CR). For these reasons, an organic light-emitting display device is currently under development as the next generation display device.
An organic light-emitting display device is a self-luminance display device. An organic light-emitting display device utilizes an organic light-emitting diode in which electrons from a cathode and holes from an anode are injected into an emitting layer, and the electrons and holes recombine to form excitons, such that light is emitted when the excitons transition from an excited state to the ground state.
Organic light-emitting display devices can be sorted into a top-emission organic light-emitting display device, a bottom-emission organic light-emitting display device and a dual-emission organic light-emitting display device depending on the direction in which light exits. Further, organic light-emitting display devices can be sorted into an active matrix organic light-emitting display device and a passive matrix organic light-emitting display device depending on the driving manner.
As display devices have higher and higher resolution, the number of pixels per unit area increases, and a high luminance is required. However, the current per area is limited by the emission structure of an organic light-emitting display device. In addition, as the amount of applied current increases, reliability of an organic light-emitting display device is degraded, and the power consumption increases.
In view of the above, a variety of structures for organic light-emitting diodes have been proposed for increasing the efficiency and lifetime of organic light-emitting diodes while reducing power consumption.
Specifically, in addition to a single-stack architecture that employs a single stack, i.e., a single light-emitting unit (EL unit), a multi-stack architecture has been proposed that employs multiple stacks, i.e., a plurality of EL units for improving efficiency and elongating lifetime.
In an organic light-emitting diode employing a multi-stack architecture including a stack of multiple EL units, an emission zone where light is emitted by recombination of electrons and holes is disposed in each of the multiple EL units. Therefore, the organic light-emitting diode having a multi-stack architecture can exhibit higher efficiency than existing organic light-emitting diodes having a single-stack architecture. In addition, it can be driven with a low current, and thus the lifetime can be improved.
However, if the amount of dopants doped into each of the plurality of EL units is not appropriate, electrons and holes are not balanced in the EL units. As a result, problems may arise in the efficiency, driving voltage and lifetime characteristics of the organic light-emitting diodes.